Method and apparatus for processing video signal

ABSTRACT

A method for decoding a video according to the present invention may comprise: deriving a spatial merge candidate for a current block, generating a merge candidate list for the current block based on the spatial merge candidate, obtaining motion information for the current block based on the merge candidate list, and performing motion compensation for the current block based on the motion information. Herein, if the current block does not have a pre-defined shape or a size equal to or greater than a pre-defined size, the spatial merge candidate of the current block may be derived based on a block which have the pre-defined shape or a size equal to or greater than the pre-defined size, the block including the current block.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage Patent Application of PCTInternational Patent Application No. PCT/KR2017/009526 (filed on Aug.31, 2017) under 35 U.S.C. § 371, which claims priority to Korean PatentApplication No. 10-2016-0112127 (filed on Aug. 31, 2016), the teachingsof which are incorporated herein in their entireties by reference.

TECHNICAL FIELD

The present invention relates to a method and an apparatus forprocessing video signal.

BACKGROUND ART

Recently, demands for high-resolution and high-quality images such ashigh definition (HD) images and ultra-high definition (UHD) images haveincreased in various application fields. However, higher resolution andquality image data has increasing amounts of data in comparison withconventional image data. Therefore, when transmitting image data byusing a medium such as conventional wired and wireless broadbandnetworks, or when storing image data by using a conventional storagemedium, costs of transmitting and storing increase. In order to solvethese problems occurring with an increase in resolution and quality ofimage data, high-efficiency image encoding/decoding techniques may beutilized.

Image compression technology includes various techniques, including: aninter-prediction technique of predicting a pixel value included in acurrent picture from a previous or subsequent picture of the currentpicture; an intra-prediction technique of predicting a pixel valueincluded in a current picture by using pixel information in the currentpicture; an entropy encoding technique of assigning a short code to avalue with a high appearance frequency and assigning a long code to avalue with a low appearance frequency; etc. Image data may beeffectively compressed by using such image compression technology, andmay be transmitted or stored.

In the meantime, with demands for high-resolution images, demands forstereographic image content, which is a new image service, have alsoincreased. A video compression technique for effectively providingstereographic image content with high resolution and ultra-highresolution is being discussed.

DISCLOSURE Technical Problem

An object of the present invention is to provide a method and anapparatus for efficiently performing a transform/inverse transform inencoding/decoding a video signal.

An object of the present invention is to provide a method and anapparatus for adaptively determining a transform type of a current blockamong a plurality of transform type candidates in encoding/decoding avideo signal.

An object of the present invention is to provide a method and anapparatus for determining transform types of a horizontal transform anda vertical transform separately in encoding/decoding a video signal.

The technical objects to be achieved by the present invention are notlimited to the above-mentioned technical problems. And, other technicalproblems that are not mentioned will be apparently understood to thoseskilled in the art from the following description.

Technical Solution

A method and an apparatus for decoding a video signal according to thepresent invention may obtain a transform coefficient of a current block,inverse quantize the transform coefficient, determine a transform setfor the current block, determine one of a plurality of transform typecandidates as a transform type of the current block, and inversetransform the inverse quantized transform coefficient based on thedetermined transform type.

A method and an apparatus for encoding a video signal according to thepresent invention may obtain a transform coefficient of a current block,inverse quantize the transform coefficient, determine a transform setfor the current block, determine one of a plurality of transform typecandidates as a transform type of the current block, and inversetransform the inverse quantized transform coefficient based on thedetermined transform type.

In the method and the apparatus for encoding/decoding a video signalaccording to the present invention, the transform set of the currentblock may be determined based on index information indicating at leastone among a plurality of transform sets.

In the method and the apparatus for encoding/decoding a video signalaccording to the present invention, at least one of a type or a numberof a transform type candidate for each of the plurality of transform setmay be different.

In the method and the apparatus for encoding/decoding a video signalaccording to the present invention, at least one of a type or a numberof a transform type candidate included in the transform set may bedetermined differently according to whether a transform skip is allowedor not.

In the method and the apparatus for encoding/decoding a video signalaccording to the present invention, the inverse transform may comprise ahorizontal transform and a vertical transform and a transform set forthe horizontal transform and a transform set for the vertical transformmay be determined independently.

In the method and the apparatus for encoding/decoding a video signalaccording to the present invention, the transform set for the horizontaltransform and the transform set for the vertical transform may bedetermined according to an intra prediction mode of the current block.

In the method and the apparatus for encoding/decoding a video signalaccording to the present invention, the transform type of the currentblock may be adaptively determined based on at least one of a size, ashape or a number of samples of the current block.

The features briefly summarized above for the present invention are onlyillustrative aspects of the detailed description of the invention thatfollows, but do not limit the scope of the invention.

Advantageous Effects

According to the present invention, a transform/inverse transform for anencoding/decoding target block can be performed efficiently.

According to the present invention, a transform type of a current blockcan be determined adaptively among a plurality of transform typecandidates.

According to the present invention, transform types of a horizontaltransform and a vertical transform can be determined separately.

The effects obtainable by the present invention are not limited to theabove-mentioned effects, and other effects not mentioned can be clearlyunderstood by those skilled in the art from the description below.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a device for encoding a videoaccording to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a device for decoding a videoaccording to an embodiment of the present invention.

FIG. 3 is a diagram illustrating an example of hierarchicallypartitioning a coding block based on a tree structure according to anembodiment of the present invention.

FIG. 4 is a diagram illustrating a partition type in which binarytree-based partitioning is allowed according to an embodiment of thepresent invention.

FIGS. 5A and 5B are diagrams illustrating an example in which only abinary tree-based partition of a predetermined type is allowed accordingto an embodiment of the present invention.

FIG. 6 is a diagram for explaining an example in which informationrelated to the allowable number of binary tree partitioning isencoded/decoded, according to an embodiment to which the presentinvention is applied.

FIG. 7 is a diagram illustrating a partition mode applicable to a codingblock according to an embodiment of the present invention.

FIG. 8 is a flowchart illustrating processes of obtaining a residualsample according to an embodiment of the present invention.

FIG. 9 is a diagram illustrating, for 33 intra prediction modes, whethera vertical transform and a horizontal transform use the same transformset.

MODE FOR INVENTION

A variety of modifications may be made to the present invention andthere are various embodiments of the present invention, examples ofwhich will now be provided with reference to drawings and described indetail. However, the present invention is not limited thereto, and theexemplary embodiments can be construed as including all modifications,equivalents, or substitutes in a technical concept and a technical scopeof the present invention. The similar reference numerals refer to thesimilar element in described the drawings.

Terms used in the specification, ‘first’, ‘second’, etc. can be used todescribe various components, but the components are not to be construedas being limited to the terms. The terms are only used to differentiateone component from other components. For example, the ‘first’ componentmay be named the ‘second’ component without departing from the scope ofthe present invention, and the ‘second’ component may also be similarlynamed the ‘first’ component. The term ‘and/or’ includes a combination ofa plurality of items or any one of a plurality of terms.

It will be understood that when an element is simply referred to asbeing ‘connected to’ or ‘coupled to’ another element without being‘directly connected to’ or ‘directly coupled to’ another element in thepresent description, it may be ‘directly connected to’ or ‘directlycoupled to’ another element or be connected to or coupled to anotherelement, having the other element intervening therebetween. In contrast,it should be understood that when an element is referred to as being“directly coupled” or “directly connected” to another element, there areno intervening elements present.

The terms used in the present specification are merely used to describeparticular embodiments, and are not intended to limit the presentinvention. An expression used in the singular encompasses the expressionof the plural, unless it has a clearly different meaning in the context.In the present specification, it is to be understood that terms such as“including”, “having”, etc. are intended to indicate the existence ofthe features, numbers, steps, actions, elements, parts, or combinationsthereof disclosed in the specification, and are not intended to precludethe possibility that one or more other features, numbers, steps,actions, elements, parts, or combinations thereof may exist or may beadded.

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.Hereinafter, the same constituent elements in the drawings are denotedby the same reference numerals, and a repeated description of the sameelements will be omitted.

FIG. 1 is a block diagram illustrating a device for encoding a videoaccording to an embodiment of the present invention.

Referring to FIG. 1, the device 100 for encoding a video may include: apicture partitioning module 110, prediction modules 120 and 125, atransform module 130, a quantization module 135, a rearrangement module160, an entropy encoding module 165, an inverse quantization module 140,an inverse transform module 145, a filter module 150, and a memory 155.

The constitutional parts shown in FIG. 1 are independently shown so asto represent characteristic functions different from each other in thedevice for encoding a video. Thus, it does not mean that eachconstitutional part is constituted in a constitutional unit of separatedhardware or software. In other words, each constitutional part includeseach of enumerated constitutional parts for convenience. Thus, at leasttwo constitutional parts of each constitutional part may be combined toform one constitutional part or one constitutional part may be dividedinto a plurality of constitutional parts to perform each function. Theembodiment where each constitutional part is combined and the embodimentwhere one constitutional part is divided are also included in the scopeof the present invention, if not departing from the essence of thepresent invention.

Also, some of constituents may not be indispensable constituentsperforming essential functions of the present invention but be selectiveconstituents improving only performance thereof. The present inventionmay be implemented by including only the indispensable constitutionalparts for implementing the essence of the present invention except theconstituents used in improving performance. The structure including onlythe indispensable constituents except the selective constituents used inimproving only performance is also included in the scope of the presentinvention.

The picture partitioning module 110 may partition an input picture intoone or more processing units. Here, the processing unit may be aprediction unit (PU), a transform unit (TU), or a coding unit (CU). Thepicture partitioning module 110 may partition one picture intocombinations of multiple coding units, prediction units, and transformunits, and may encode a picture by selecting one combination of codingunits, prediction units, and transform units with a predeterminedcriterion (e.g., cost function).

For example, one picture may be partitioned into multiple coding units.A recursive tree structure, such as a quad tree structure, may be usedto partition a picture into coding units. A coding unit which ispartitioned into other coding units with one picture or a largest codingunit as a root may be partitioned with child nodes corresponding to thenumber of partitioned coding units. A coding unit which is no longerpartitioned by a predetermined limitation serves as a leaf node. Thatis, when it is assumed that only square partitioning is possible for onecoding unit, one coding unit may be partitioned into four other codingunits at most.

Hereinafter, in the embodiment of the present invention, the coding unitmay mean a unit performing encoding, or a unit performing decoding.

A prediction unit may be one of partitions partitioned into a square ora rectangular shape having the same size in a single coding unit, or aprediction unit may be one of partitions partitioned so as to have adifferent shape/size in a single coding unit.

When a prediction unit subjected to intra prediction is generated basedon a coding unit and the coding unit is not the smallest coding unit,intra prediction may be performed without partitioning the coding unitinto multiple prediction units N×N.

The prediction modules 120 and 125 may include an inter predictionmodule 120 performing inter prediction and an intra prediction module125 performing intra prediction. Whether to perform inter prediction orintra prediction for the prediction unit may be determined, and detailedinformation (e.g., an intra prediction mode, a motion vector, areference picture, etc.) according to each prediction method may bedetermined. Here, the processing unit subjected to prediction may bedifferent from the processing unit for which the prediction method anddetailed content is determined. For example, the prediction method, theprediction mode, etc. may be determined by the prediction unit, andprediction may be performed by the transform unit. A residual value(residual block) between the generated prediction block and an originalblock may be input to the transform module 130. Also, prediction modeinformation, motion vector information, etc. used for prediction may beencoded with the residual value by the entropy encoding module 165 andmay be transmitted to a device for decoding a video. When a particularencoding mode is used, it is possible to transmit to a device fordecoding video by encoding the original block as it is withoutgenerating the prediction block through the prediction modules 120 and125.

The inter prediction module 120 may predict the prediction unit based oninformation of at least one of a previous picture or a subsequentpicture of the current picture, or may predict the prediction unit basedon information of some encoded regions in the current picture, in somecases. The inter prediction module 120 may include a reference pictureinterpolation module, a motion prediction module, and a motioncompensation module.

The reference picture interpolation module may receive reference pictureinformation from the memory 155 and may generate pixel information of aninteger pixel or less then the integer pixel from the reference picture.In the case of luma pixels, an 8-tap DCT-based interpolation filterhaving different filter coefficients may be used to generate pixelinformation of an integer pixel or less than an integer pixel in a unitof a ¼ pixel. In the case of chroma signals, a 4-tap DCT-basedinterpolation filter having different filter coefficient may be used togenerate pixel information of an integer pixel or less than an integerpixel in a unit of a ⅛ pixel.

The motion prediction module may perform motion prediction based on thereference picture interpolated by the reference picture interpolationmodule. As methods for calculating a motion vector, various methods,such as a full search-based block matching algorithm (FBMA), a threestep search (TSS), a new three-step search algorithm (NTS), etc., may beused. The motion vector may have a motion vector value in a unit of a ½pixel or a ¼ pixel based on an interpolated pixel. The motion predictionmodule may predict a current prediction unit by changing the motionprediction method. As motion prediction methods, various methods, suchas a skip method, a merge method, an AMVP (Advanced Motion VectorPrediction) method, an intra block copy method, etc., may be used.

The intra prediction module 125 may generate a prediction unit based onreference pixel information neighboring to a current block which ispixel information in the current picture. When the neighboring block ofthe current prediction unit is a block subjected to inter prediction andthus a reference pixel is a pixel subjected to inter prediction, thereference pixel included in the block subjected to inter prediction maybe replaced with reference pixel information of a neighboring blocksubjected to intra prediction. That is, when a reference pixel is notavailable, at least one reference pixel of available reference pixelsmay be used instead of unavailable reference pixel information.

Prediction modes in intra prediction may include a directionalprediction mode using reference pixel information depending on aprediction direction and a non-directional prediction mode not usingdirectional information in performing prediction. A mode for predictingluma information may be different from a mode for predicting chromainformation, and in order to predict the chroma information, intraprediction mode information used to predict luma information orpredicted luma signal information may be utilized.

In performing intra prediction, when the size of the prediction unit isthe same as the size of the transform unit, intra prediction may beperformed on the prediction unit based on pixels positioned at the left,the top left, and the top of the prediction unit. However, in performingintra prediction, when the size of the prediction unit is different fromthe size of the transform unit, intra prediction may be performed usinga reference pixel based on the transform unit. Also, intra predictionusing N×N partitioning may be used for only the smallest coding unit.

In the intra prediction method, a prediction block may be generatedafter applying an AIS (Adaptive Intra Smoothing) filter to a referencepixel depending on the prediction modes. The type of the AIS filterapplied to the reference pixel may vary. In order to perform the intraprediction method, an intra prediction mode of the current predictionunit may be predicted from the intra prediction mode of the predictionunit neighboring to the current prediction unit. In prediction of theprediction mode of the current prediction unit by using mode informationpredicted from the neighboring prediction unit, when the intraprediction mode of the current prediction unit is the same as the intraprediction mode of the neighboring prediction unit, informationindicating that the prediction modes of the current prediction unit andthe neighboring prediction unit are equal to each other may betransmitted using predetermined flag information. When the predictionmode of the current prediction unit is different from the predictionmode of the neighboring prediction unit, entropy encoding may beperformed to encode prediction mode information of the current block.

Also, a residual block including information on a residual value whichis a different between the prediction unit subjected to prediction andthe original block of the prediction unit may be generated based onprediction units generated by the prediction modules 120 and 125. Thegenerated residual block may be input to the transform module 130.

The transform module 130 may transform the residual block including theinformation on the residual value between the original block and theprediction unit generated by the prediction modules 120 and 125 by usinga transform method, such as discrete cosine transform (DCT), discretesine transform (DST), and KLT. Whether to apply DCT, DST, or KLT inorder to transform the residual block may be determined based on intraprediction mode information of the prediction unit used to generate theresidual block.

The quantization module 135 may quantize values transformed to afrequency domain by the transform module 130. Quantization coefficientsmay vary depending on the block or importance of a picture. The valuescalculated by the quantization module 135 may be provided to the inversequantization module 140 and the rearrangement module 160.

The rearrangement module 160 may rearrange coefficients of quantizedresidual values.

The rearrangement module 160 may change a coefficient in the form of atwo-dimensional block into a coefficient in the form of aone-dimensional vector through a coefficient scanning method. Forexample, the rearrangement module 160 may scan from a DC coefficient toa coefficient in a high frequency domain using a zigzag scanning methodso as to change the coefficients to be in the form of one-dimensionalvectors. Depending on the size of the transform unit and the intraprediction mode, vertical direction scanning where coefficients in theform of two-dimensional blocks are scanned in the column direction orhorizontal direction scanning where coefficients in the form oftwo-dimensional blocks are scanned in the row direction may be usedinstead of zigzag scanning. That is, which scanning method among zigzagscanning, vertical direction scanning, and horizontal direction scanningis used may be determined depending on the size of the transform unitand the intra prediction mode.

The entropy encoding module 165 may perform entropy encoding based onthe values calculated by the rearrangement module 160. Entropy encodingmay use various encoding methods, for example, exponential Golombcoding, context-adaptive variable length coding (CAVLC), andcontext-adaptive binary arithmetic coding (CABAC).

The entropy encoding module 165 may encode a variety of information,such as residual value coefficient information and block typeinformation of the coding unit, prediction mode information, partitionunit information, prediction unit information, transform unitinformation, motion vector information, reference frame information,block interpolation information, filtering information, etc. from therearrangement module 160 and the prediction modules 120 and 125.

The entropy encoding module 165 may entropy encode the coefficients ofthe coding unit input from the rearrangement module 160.

The inverse quantization module 140 may inversely quantize the valuesquantized by the quantization module 135 and the inverse transformmodule 145 may inversely transform the values transformed by thetransform module 130. The residual value generated by the inversequantization module 140 and the inverse transform module 145 may becombined with the prediction unit predicted by a motion estimationmodule, a motion compensation module, and the intra prediction module ofthe prediction modules 120 and 125 such that a reconstructed block canbe generated.

The filter module 150 may include at least one of a deblocking filter,an offset correction unit, and an adaptive loop filter (ALF).

The deblocking filter may remove block distortion that occurs due toboundaries between the blocks in the reconstructed picture. In order todetermine whether to perform deblocking, the pixels included in severalrows or columns in the block may be a basis of determining whether toapply the deblocking filter to the current block. When the deblockingfilter is applied to the block, a strong filter or a weak filter may beapplied depending on required deblocking filtering strength. Also, inapplying the deblocking filter, horizontal direction filtering andvertical direction filtering may be processed in parallel.

The offset correction module may correct offset with the originalpicture in a unit of a pixel in the picture subjected to deblocking. Inorder to perform the offset correction on a particular picture, it ispossible to use a method of applying offset in consideration of edgeinformation of each pixel or a method of partitioning pixels of apicture into the predetermined number of regions, determining a regionto be subjected to perform offset, and applying the offset to thedetermined region.

Adaptive loop filtering (ALF) may be performed based on the valueobtained by comparing the filtered reconstructed picture and theoriginal picture. The pixels included in the picture may be divided intopredetermined groups, a filter to be applied to each of the groups maybe determined, and filtering may be individually performed for eachgroup. Information on whether to apply ALF and a luma signal may betransmitted by coding units (CU). The shape and filter coefficient of afilter for ALF may vary depending on each block. Also, the filter forALF in the same shape (fixed shape) may be applied regardless ofcharacteristics of the application target block.

The memory 155 may store the reconstructed block or picture calculatedthrough the filter module 150. The stored reconstructed block or picturemay be provided to the prediction modules 120 and 125 in performinginter prediction.

FIG. 2 is a block diagram illustrating a device for decoding a videoaccording to an embodiment of the present invention.

Referring to FIG. 2, the device 200 for decoding a video may include: anentropy decoding module 210, a rearrangement module 215, an inversequantization module 220, an inverse transform module 225, predictionmodules 230 and 235, a filter module 240, and a memory 245.

When a video bitstream is input from the device for encoding a video,the input bitstream may be decoded according to an inverse process ofthe device for encoding a video.

The entropy decoding module 210 may perform entropy decoding accordingto an inverse process of entropy encoding by the entropy encoding moduleof the device for encoding a video. For example, corresponding to themethods performed by the device for encoding a video, various methods,such as exponential Golomb coding, context-adaptive variable lengthcoding (CAVLC), and context-adaptive binary arithmetic coding (CABAC)may be applied.

The entropy decoding module 210 may decode information on intraprediction and inter prediction performed by the device for encoding avideo.

The rearrangement module 215 may perform rearrangement on the bitstreamentropy decoded by the entropy decoding module 210 based on therearrangement method used in the device for encoding a video. Therearrangement module may reconstruct and rearrange the coefficients inthe form of one-dimensional vectors to the coefficient in the form oftwo-dimensional blocks. The rearrangement module 215 may receiveinformation related to coefficient scanning performed in the device forencoding a video and may perform rearrangement via a method of inverselyscanning the coefficients based on the scanning order performed in thedevice for encoding a video.

The inverse quantization module 220 may perform inverse quantizationbased on a quantization parameter received from the device for encodinga video and the rearranged coefficients of the block.

The inverse transform module 225 may perform the inverse transform,i.e., inverse DCT, inverse DST, and inverse KLT, which is the inverseprocess of transform, i.e., DCT, DST, and KLT, performed by thetransform module on the quantization result by the device for encoding avideo. Inverse transform may be performed based on a transfer unitdetermined by the device for encoding a video. The inverse transformmodule 225 of the device for decoding a video may selectively performtransform schemes (e.g., DCT, DST, and KLT) depending on multiple piecesof information, such as the prediction method, the size of the currentblock, the prediction direction, etc.

The prediction modules 230 and 235 may generate a prediction block basedon information on prediction block generation received from the entropydecoding module 210 and previously decoded block or picture informationreceived from the memory 245.

As described above, like the operation of the device for encoding avideo, in performing intra prediction, when the size of the predictionunit is the same as the size of the transform unit, intra prediction maybe performed on the prediction unit based on the pixels positioned atthe left, the top left, and the top of the prediction unit. Inperforming intra prediction, when the size of the prediction unit isdifferent from the size of the transform unit, intra prediction may beperformed using a reference pixel based on the transform unit. Also,intra prediction using N×N partitioning may be used for only thesmallest coding unit.

The prediction modules 230 and 235 may include a prediction unitdetermination module, an inter prediction module, and an intraprediction module. The prediction unit determination module may receivea variety of information, such as prediction unit information,prediction mode information of an intra prediction method, informationon motion prediction of an inter prediction method, etc. from theentropy decoding module 210, may divide a current coding unit intoprediction units, and may determine whether inter prediction or intraprediction is performed on the prediction unit. By using informationrequired in inter prediction of the current prediction unit receivedfrom the device for encoding a video, the inter prediction module 230may perform inter prediction on the current prediction unit based oninformation of at least one of a previous picture or a subsequentpicture of the current picture including the current prediction unit.Alternatively, inter prediction may be performed based on information ofsome pre-reconstructed regions in the current picture including thecurrent prediction unit.

In order to perform inter prediction, it may be determined for thecoding unit which of a skip mode, a merge mode, an AMVP mode, and aninter block copy mode is used as the motion prediction method of theprediction unit included in the coding unit.

The intra prediction module 235 may generate a prediction block based onpixel information in the current picture. When the prediction unit is aprediction unit subjected to intra prediction, intra prediction may beperformed based on intra prediction mode information of the predictionunit received from the device for encoding a video. The intra predictionmodule 235 may include an adaptive intra smoothing (AIS) filter, areference pixel interpolation module, and a DC filter. The AIS filterperforms filtering on the reference pixel of the current block, andwhether to apply the filter may be determined depending on theprediction mode of the current prediction unit. AIS filtering may beperformed on the reference pixel of the current block by using theprediction mode of the prediction unit and AIS filter informationreceived from the device for encoding a video. When the prediction modeof the current block is a mode where AIS filtering is not performed, theAIS filter may not be applied.

When the prediction mode of the prediction unit is a prediction mode inwhich intra prediction is performed based on the pixel value obtained byinterpolating the reference pixel, the reference pixel interpolationmodule may interpolate the reference pixel to generate the referencepixel of an integer pixel or less than an integer pixel. When theprediction mode of the current prediction unit is a prediction mode inwhich a prediction block is generated without interpolation thereference pixel, the reference pixel may not be interpolated. The DCfilter may generate a prediction block through filtering when theprediction mode of the current block is a DC mode.

The reconstructed block or picture may be provided to the filter module240. The filter module 240 may include the deblocking filter, the offsetcorrection module, and the ALF.

Information on whether or not the deblocking filter is applied to thecorresponding block or picture and information on which of a strongfilter and a weak filter is applied when the deblocking filter isapplied may be received from the device for encoding a video. Thedeblocking filter of the device for decoding a video may receiveinformation on the deblocking filter from the device for encoding avideo, and may perform deblocking filtering on the corresponding block.

The offset correction module may perform offset correction on thereconstructed picture based on the type of offset correction and offsetvalue information applied to a picture in performing encoding.

The ALF may be applied to the coding unit based on information onwhether to apply the ALF, ALF coefficient information, etc. receivedfrom the device for encoding a video. The ALF information may beprovided as being included in a particular parameter set.

The memory 245 may store the reconstructed picture or block for use as areference picture or block, and may provide the reconstructed picture toan output module.

As described above, in the embodiment of the present invention, forconvenience of explanation, the coding unit is used as a termrepresenting a unit for encoding, but the coding unit may serve as aunit performing decoding as well as encoding.

In addition, a current block may represent a target block to beencoded/decoded. And, the current block may represent a coding treeblock (or a coding tree unit), a coding block (or a coding unit), atransform block (or a transform unit), a prediction block (or aprediction unit), or the like depending on an encoding/decoding step.

A picture may be encoded/decoded by divided into base blocks having asquare shape or a non-square shape. At this time, the base block may bereferred to as a coding tree unit. The coding tree unit may be definedas a coding unit of the largest size allowed within a sequence or aslice. Information regarding whether the coding tree unit has a squareshape or has a non-square shape or information regarding a size of thecoding tree unit may be signaled through a sequence parameter set, apicture parameter set, or a slice header. The coding tree unit may bedivided into smaller size partitions. At this time, if it is assumedthat a depth of a partition generated by dividing the coding tree unitis 1, a depth of a partition generated by dividing the partition havingdepth 1 may be defined as 2. That is, a partition generated by dividinga partition having a depth k in the coding tree unit may be defined ashaving a depth k+1.

A partition of arbitrary size generated by dividing a coding tree unitmay be defined as a coding unit. The coding unit may be recursivelydivided or divided into base units for performing prediction,quantization, transform, or in-loop filtering, and the like. Forexample, a partition of arbitrary size generated by dividing the codingunit may be defined as a coding unit, or may be defined as a transformunit or a prediction unit, which is a base unit for performingprediction, quantization, transform or in-loop filtering and the like.

Partitioning of a coding tree unit or a coding unit may be performedbased on at least one of a vertical line and a horizontal line. Inaddition, the number of vertical lines or horizontal lines partitioningthe coding tree unit or the coding unit may be at least one or more. Forexample, the coding tree unit or the coding unit may be divided into twopartitions using one vertical line or one horizontal line, or the codingtree unit or the coding unit may be divided into three partitions usingtwo vertical lines or two horizontal lines. Alternatively, the codingtree unit or the coding unit may be partitioned into four partitionshaving a length and a width of ½ by using one vertical line and onehorizontal line.

When a coding tree unit or a coding unit is divided into a plurality ofpartitions using at least one vertical line or at least one horizontalline, the partitions may have a uniform size or a different size.Alternatively, any one partition may have a different size from theremaining partitions.

In the embodiments described below, it is assumed that a coding treeunit or a coding unit is divided into a quad tree structure or a binarytree structure. However, it is also possible to divide a coding treeunit or a coding unit using a larger number of vertical lines or alarger number of horizontal lines.

FIG. 3 is a diagram illustrating an example of hierarchicallypartitioning a coding block based on a tree structure according to anembodiment of the present invention.

An input video signal is decoded in predetermined block units. Such adefault unit for decoding the input video signal is a coding block. Thecoding block may be a unit performing intra/inter prediction, transform,and quantization. In addition, a prediction mode (e.g., intra predictionmode or inter prediction mode) is determined in a unit of a codingblock, and the prediction blocks included in the coding block may sharethe determined prediction mode. The coding block may be a square ornon-square block having an arbitrary size in a range of 8×8 to 64×64, ormay be a square or non-square block having a size of 128×128, 256×256,or more.

Specifically, the coding block may be hierarchically partitioned basedon at least one of a quad tree and a binary tree. Here, quad tree-basedpartitioning may mean that a 2N×2N coding block is partitioned into fourN×N coding blocks, and binary tree-based partitioning may mean that onecoding block is partitioned into two coding blocks. Even if the binarytree-based partitioning is performed, a square-shaped coding block mayexist in the lower depth.

Binary tree-based partitioning may be symmetrically or asymmetricallyperformed. The coding block partitioned based on the binary tree may bea square block or a non-square block, such as a rectangular shape. Forexample, a partition type in which the binary tree-based partitioning isallowed may comprise at least one of a symmetric type of 2N×N(horizontal directional non-square coding unit) or N×2N (verticaldirection non-square coding unit), asymmetric type of nL×2N, nR×2N,2N×nU, or 2N×nD.

Binary tree-based partitioning may be limitedly allowed to one of asymmetric or an asymmetric type partition. In this case, constructingthe coding tree unit with square blocks may correspond to quad tree CUpartitioning, and constructing the coding tree unit with symmetricnon-square blocks may correspond to binary tree partitioning.Constructing the coding tree unit with square blocks and symmetricnon-square blocks may correspond to quad and binary tree CUpartitioning.

Binary tree-based partitioning may be performed on a coding block wherequad tree-based partitioning is no longer performed. Quad tree-basedpartitioning may no longer be performed on the coding block partitionedbased on the binary tree.

Furthermore, partitioning of a lower depth may be determined dependingon a partition type of an upper depth. For example, if binary tree-basedpartitioning is allowed in two or more depths, only the same type as thebinary tree partitioning of the upper depth may be allowed in the lowerdepth. For example, if the binary tree-based partitioning in the upperdepth is performed with 2N×N type, the binary tree-based partitioning inthe lower depth is also performed with 2N×N type. Alternatively, if thebinary tree-based partitioning in the upper depth is performed with N×2Ntype, the binary tree-based partitioning in the lower depth is alsoperformed with N×2N type.

On the contrary, it is also possible to allow, in a lower depth, only atype different from a binary tree partitioning type of an upper depth.

It may be possible to limit only a specific type of binary tree basedpartitioning to be used for sequence, slice, coding tree unit, or codingunit. As an example, only 2N×N type or N×2N type of binary tree-basedpartitioning may be allowed for the coding tree unit. An availablepartition type may be predefined in an encoder or a decoder. Orinformation on available partition type or on unavailable partition typeon may be encoded and then signaled through a bitstream.

FIGS. 5A and 5B are diagrams illustrating an example in which only aspecific type of binary tree-based partitioning is allowed. FIG. 5Ashows an example in which only N×2N type of binary tree-basedpartitioning is allowed, and FIG. 5B shows an example in which only 2N×Ntype of binary tree-based partitioning is allowed. In order to implementadaptive partitioning based on the quad tree or binary tree, informationindicating quad tree-based partitioning, information on the size/depthof the coding block that quad tree-based partitioning is allowed,information indicating binary tree-based partitioning, information onthe size/depth of the coding block that binary tree-based partitioningis allowed, information on the size/depth of the coding block thatbinary tree-based partitioning is not allowed, information on whetherbinary tree-based partitioning is performed in a vertical direction or ahorizontal direction, etc. may be used.

In addition, information on the number of times a binary treepartitioning is allowed, a depth at which the binary tree partitioningis allowed, or the number of the depths at which the binary treepartitioning is allowed may be obtained for a coding tree unit or aspecific coding unit. The information may be encoded in a unit of acoding tree unit or a coding unit, and may be transmitted to a decoderthrough a bitstream.

For example, a syntax ‘max_binary_depth_idx_minus1’ indicating a maximumdepth at which binary tree partitioning is allowed may beencoded/decoded through a bitstream. In this case,max_binary_depth_idx_minus+1 may indicate the maximum depth at which thebinary tree partitioning is allowed.

Referring to the example shown in FIG. 6, in FIG. 6, the binary treepartitioning has been performed for a coding unit having a depth of 2and a coding unit having a depth of 3. Accordingly, at least one ofinformation indicating the number of times the binary tree partitioningin the coding tree unit has been performed (i.e., 2 times), informationindicating the maximum depth which the binary tree partitioning has beenallowed in the coding tree unit (i.e., depth 3), or the number of depthsin which the binary tree partitioning has been performed in the codingtree unit (i.e., 2 (depth 2 and depth 3)) may be encoded/decoded througha bitstream.

As another example, at least one of information on the number of timesthe binary tree partitioning is permitted, the depth at which the binarytree partitioning is allowed, or the number of the depths at which thebinary tree partitioning is allowed may be obtained for each sequence oreach slice. For example, the information may be encoded in a unit of asequence, a picture, or a slice unit and transmitted through abitstream. Accordingly, at least one of the number of the binary treepartitioning in a first slice, the maximum depth in which the binarytree partitioning is allowed in the first slice, or the number of depthsin which the binary tree partitioning is performed in the first slicemay be difference from a second slice. For example, in the first slice,binary tree partitioning may be permitted for only one depth, while inthe second slice, binary tree partitioning may be permitted for twodepths.

As another example, the number of times the binary tree partitioning ispermitted, the depth at which the binary tree partitioning is allowed,or the number of depths at which the binary tree partitioning is allowedmay be set differently according to a time level identifier (TemporalID)of a slice or a picture. Here, the temporal level identifier(TemporalID) is used to identify each of a plurality of layers of videohaving a scalability of at least one of view, spatial, temporal orquality.

As shown in FIG. 3, the first coding block 300 with the partition depth(split depth) of k may be partitioned into multiple second coding blocksbased on the quad tree. For example, the second coding blocks 310 to 340may be square blocks having the half width and the half height of thefirst coding block, and the partition depth of the second coding blockmay be increased to k+1.

The second coding block 310 with the partition depth of k+1 may bepartitioned into multiple third coding blocks with the partition depthof k+2. Partitioning of the second coding block 310 may be performed byselectively using one of the quad tree and the binary tree depending ona partitioning method. Here, the partitioning method may be determinedbased on at least one of the information indicating quad tree-basedpartitioning and the information indicating binary tree-basedpartitioning.

When the second coding block 310 is partitioned based on the quad tree,the second coding block 310 may be partitioned into four third codingblocks 310 a having the half width and the half height of the secondcoding block, and the partition depth of the third coding block 310 amay be increased to k+2. In contrast, when the second coding block 310is partitioned based on the binary tree, the second coding block 310 maybe partitioned into two third coding blocks. Here, each of two thirdcoding blocks may be a non-square block having one of the half width andthe half height of the second coding block, and the partition depth maybe increased to k+2. The second coding block may be determined as anon-square block of a horizontal direction or a vertical directiondepending on a partitioning direction, and the partitioning directionmay be determined based on the information on whether binary tree-basedpartitioning is performed in a vertical direction or a horizontaldirection.

In the meantime, the second coding block 310 may be determined as a leafcoding block that is no longer partitioned based on the quad tree or thebinary tree. In this case, the leaf coding block may be used as aprediction block or a transform block.

Like partitioning of the second coding block 310, the third coding block310 a may be determined as a leaf coding block, or may be furtherpartitioned based on the quad tree or the binary tree.

In the meantime, the third coding block 310 b partitioned based on thebinary tree may be further partitioned into coding blocks 310 b-2 of avertical direction or coding blocks 310 b-3 of a horizontal directionbased on the binary tree, and the partition depth of the relevant codingblocks may be increased to k+3. Alternatively, the third coding block310 b may be determined as a leaf coding block 310 b-1 that is no longerpartitioned based on the binary tree. In this case, the coding block 310b-1 may be used as a prediction block or a transform block. However, theabove partitioning process may be limitedly performed based on at leastone of the information on the size/depth of the coding block that quadtree-based partitioning is allowed, the information on the size/depth ofthe coding block that binary tree-based partitioning is allowed, and theinformation on the size/depth of the coding block that binary tree-basedpartitioning is not allowed.

A number of a candidate that represent a size of a coding block may belimited to a predetermined number, or a size of a coding block in apredetermined unit may have a fixed value. As an example, the size ofthe coding block in a sequence or in a picture may be limited to have256×256, 128×128, or 32×32. Information indicating the size of thecoding block in the sequence or in the picture may be signaled through asequence header or a picture header.

As a result of partitioning based on a quad tree and a binary tree, acoding unit may be represented as square or rectangular shape of anarbitrary size.

A coding block is encoded using at least one of a skip mode, intraprediction, inter prediction, or a skip method. Once a coding block isdetermined, a prediction block may be determined through predictivepartitioning of the coding block. The predictive partitioning of thecoding block may be performed by a partition mode (Part mode) indicatinga partition type of the coding block. A size or a shape of theprediction block may be determined according to the partition mode ofthe coding block. For example, a size of a prediction block determinedaccording to the partition mode may be equal to or smaller than a sizeof a coding block.

FIG. 7 is a diagram illustrating a partition mode that may be applied toa coding block when the coding block is encoded by inter prediction.

When a coding block is encoded by inter prediction, one of 8partitioning modes may be applied to the coding block, as in the exampleshown in FIG. 4.

When a coding block is encoded by intra prediction, a partition modePART_2N×2N or a partition mode PART_N×N may be applied to the codingblock.

PART_N×N may be applied when a coding block has a minimum size. Here,the minimum size of the coding block may be pre-defined in an encoderand a decoder. Or, information regarding the minimum size of the codingblock may be signaled via a bitstream. For example, the minimum size ofthe coding block may be signaled through a slice header, so that theminimum size of the coding block may be defined per slice.

In general, a prediction block may have a size from 64×64 to 4×4.However, when a coding block is encoded by inter prediction, it may berestricted that the prediction block does not have a 4×4 size in orderto reduce memory bandwidth when performing motion compensation.

FIG. 8 is a flowchart illustrating processes of obtaining a residualsample according to an embodiment of the present invention.

First, a residual coefficient of a current block may be obtained S810.The decoder may obtain the residual coefficient through a coefficientscanning method. For example, the decoder may perform coefficientscanning using a diagonal scan, a zigzag scan, an up-right scan, avertical scan, or a horizontal scan, and thereby obtain residualcoefficients in a shape of a two-dimensional block.

Inverse quantization may be performed for the residual coefficient ofthe current block S820.

It is possible to determine whether to skip an inverse transform on thedequantized residual coefficient of the current block S830.Specifically, the decoder may determine whether to skip the inversetransform on at least one of a horizontal direction or a verticaldirection of the current block. When it is determined to apply theinverse transform on at least one of the horizontal direction or thevertical direction of the current block, a residual sample of thecurrent block may be obtained by inverse transforming the dequantizedresidual coefficient of the current block S840. Here, the inversetransform may be performed using at least one of DCT, DST, and KLT.

When the inverse transform is skipped in both the horizontal directionand the vertical direction of the current block, the inverse transformis not performed in the horizontal direction and the vertical directionof the current block. In this case, the residual sample of the currentblock may be obtained by scaling the dequantized residual coefficientwith a predetermined value S850.

Skipping the inverse transform on the horizontal direction means thatthe inverse transform is not performed on the horizontal direction butthe inverse transform is performed on the vertical direction. At thistime, scaling may be performed in the horizontal direction.

Skipping the inverse transform on the vertical direction means that theinverse transform is not performed on the vertical direction but theinverse transform is performed on the horizontal direction. At thistime, scaling may be performed in the vertical direction.

It may be determined whether or not an inverse transform skip techniquemay be used for the current block depending on a partition type of thecurrent block. For example, if the current block is generated through abinary tree-based partitioning, the inverse transform skip scheme may berestricted for the current block. Accordingly, when the current block isgenerated through the binary tree-based partitioning, the residualsample of the current block may be obtained by inverse transforming thecurrent block. In addition, when the current block is generated throughbinary tree-based partitioning, encoding/decoding of informationindicating whether or not the inverse transform is skipped (e.g.,transform_skip_flag) may be omitted.

Alternatively, when the current block is generated through binarytree-based partitioning, it is possible to limit the inverse transformskip scheme to at least one of the horizontal direction or the verticaldirection. Here, the direction in which the inverse transform skipscheme is limited may be determined based on information decoded fromthe bitstream, or may be adaptively determined based on at least one ofa size of the current block, a shape of the current block, or an intraprediction mode of the current block.

For example, when the current block is a non-square block having a widthgreater than a height, the inverse transform skip scheme may be allowedonly in the vertical direction and restricted in the horizontaldirection. That is, when the current block is 2N×N, the inversetransform is performed in the horizontal direction of the current block,and the inverse transform may be selectively performed in the verticaldirection.

On the other hand, when the current block is a non-square block having aheight greater than a width, the inverse transform skip scheme may beallowed only in the horizontal direction and restricted in the verticaldirection. That is, when the current block is N×2N, the inversetransform is performed in the vertical direction of the current block,and the inverse transform may be selectively performed in the horizontaldirection.

In contrast to the above example, when the current block is a non-squareblock having a width greater than a height, the inverse transform skipscheme may be allowed only in the horizontal direction, and when thecurrent block is a non-square block having a height greater than awidth, the inverse transform skip scheme may be allowed only in thevertical direction.

Information indicating whether or not to skip the inverse transform withrespect to the horizontal direction or information indicating whether toskip the inverse transformation with respect to the vertical directionmay be signaled through a bitstream. For example, the informationindicating whether or not to skip the inverse transform on thehorizontal direction is a 1-bit flag, ‘hor_transform_skip_flag’, andinformation indicating whether to skip the inverse transform on thevertical direction is a 1-bit flag, ‘ver_transform_skip_flag’. Theencoder may encode at least one of ‘hor_transform_skip_flag’ or‘ver_transform_skip_flag’ according to the shape of the current block.Further, the decoder may determine whether or not the inverse transformon the horizontal direction or on the vertical direction is skipped byusing at least one of “hor_transform_skip_flag” or“ver_transform_skip_flag”.

It may be set to skip the inverse transform for any one direction of thecurrent block depending on a partition type of the current block. Forexample, if the current block is generated through a binary tree-basedpartitioning, the inverse transform on the horizontal direction orvertical direction may be skipped. That is, if the current block isgenerated by binary tree-based partitioning, it may be determined thatthe inverse transform for the current block is skipped on at least oneof a horizontal direction or a vertical direction withoutencoding/decoding information (e.g., transform_skip_flag,hor_transform_skip_flag, ver_transform_skip_flag) indicating whether ornot the inverse transform of the current block is skipped.

If it is determined to apply the inverse transform to the current block,a transform type may be determined and the inverse transform may beperformed using the determined transform type. The transform type of thecurrent block (e.g., a transform block or a coding block) may bedetermined based on at least one of a size or an encoding mode of thecurrent block. Here, the encoding mode may indicate whether a predictionblock corresponding to the coding block or the transform block isencoded in intra mode or inter mode.

For example, the inverse transform for a block of 4×4 encoded in theintra mode may be performed by using DST (specifically, DST-VII), andthe inverse transform for a block other than the block may be performedby using DCT (specifically, DCT-II).

DST-VII may be defined as matrix A₄ of Equation 1. The inverse transformof DST-VII may be defined as A₄ ^(T).

$\begin{matrix}{A_{4} = \begin{bmatrix}29 & 55 & 74 & 84 \\74 & 74 & 0 & {- 74} \\84 & {- 29} & {- 74} & 55 \\55 & {- 84} & 74 & {- 29}\end{bmatrix}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

The DCT-II for a block of 8×8 may be defined as matrix T₈ of Equation 2.The inverse transform of DCT-II may be defined as T_(B) ^(T).

$\begin{matrix}{T_{8} = \begin{bmatrix}64 & 64 & 64 & 64 & 64 & 64 & 64 & 64 \\89 & 75 & 50 & 18 & {- 18} & {- 50} & {- 75} & {- 89} \\83 & 36 & {- 36} & {- 83} & {- 83} & {- 36} & 36 & 83 \\75 & {- 18} & {- 89} & {- 50} & 50 & 89 & 18 & {- 75} \\64 & {- 64} & {- 64} & 64 & 64 & {- 64} & {- 64} & 64 \\50 & {- 89} & 18 & 75 & {- 75} & {- 18} & 89 & {- 50} \\36 & {- 83} & 83 & {- 36} & {- 36} & 83 & {- 83} & 36 \\18 & {- 50} & 75 & {- 89} & 89 & {- 75} & 50 & {- 18}\end{bmatrix}} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack\end{matrix}$

A condition for selecting the transform type may be set differently on aunit of a sequence, a slice or a block. For example, in slice 0, DST isapplied to a transform block of 4×4 encoded in the intra mode, whereasin slice 0, DST is applied to a transform block of 8×8 or smallerencoded in the intra mode.

As another example, the transform type of the current block may beadaptively determined based on at least one of an intra prediction modeof the current block or the number of samples included in the currentblock. At this time, the number of samples used as a reference forselecting the transform type may have a fixed value or may be determinedthrough information signaled via the bitstream. The information may besignaled via a block level, a slice header, or a picture parameter set.

For example, DST may be applied only when the current block includes 16or less samples and when the current block is encoded in the intra mode,and DCT may be applied in other cases. Specifically, DST may be appliedto a block of 4×4, 2×8 or 8×2 encoded by the intra prediction, and DCTmay be applied to a block other than the block.

Alternatively, the transform type of the current block may be determinedfrom transform set candidates included in a transform set. At this time,different transform sets can be used in a unit of a coding block or atransform block. Alternatively, a plurality of transform blocks includedin a predetermined coding block may share the same transform set. Todetermine the transform set, index information for identifying thetransform set may be signaled in a unit of a coding block or a transformblock. Alternatively, the transform set of the current block may beadaptively determined according to a size, a shape, an encoding mode, anintra prediction mode, the number of samples of the current block, orthe like.

The transform set may include a plurality of transform type candidatesthat may be selectively used according to the shape, the size, or thenumber of samples of the transform block (or the coding block). At thistime, at least one of the number or types of transform type candidatesincluded in transform sets may be different.

Table 1 is a chart depicting transform sets including differenttransform type candidates.

TABLE 1 Transform Transform Transform set Index candidates 0 candidates1 0 DST-VII DCT-II 1 DST-VII DST-I 2 DST-VII DCT-VIII

In Table 1, it is illustrated that the number of transform typecandidates included in the transform set is two. It is also possiblethat the transform set includes one, three, four or more transform typecandidates.

In addition, the number of transform type candidates included in atleast one of the transform sets may be different from the number oftransform type candidates included in another transform set. The numberof maximum transform type candidates included in the transform set maybe signaled in a slice or a sequence header.

The transform type of the current block may be determined to be at leastone of the transform type candidates included in the transform set. Atthis time, the transform type of the transform block may be determinedbased on a size, an encoding mode, an intra prediction mode, the numberof samples of the transform block or the coding block, or the like.Here, the intra prediction mode of the transform block may be the intraprediction mode of the prediction block or the coding blockcorresponding to the transform block.

For example, when transform set index 0 is determined as the transformset of the current block, if the current block is a 4×4 block encoded inthe intra mode, transform type candidate 0, i.e., DST-VII is used, andif the current block does not satisfy the above condition, transformtype candidate 1, i.e. DCT-II, is used.

Alternatively, when transform set index 2 is determined as the transformset of the current block, if the current block is 4×4 or 8×8 blockencoded in the intra mode, transform type candidate 0, i.e., DST-VII isapplied, and if the current block does not satisfy the above condition,transform type candidate 1, i.e., DCT-VIII, is used.

According to a size of the coding block, a condition for selecting thetransform type candidate of the transform block may be set differently.For example, when the size of the coding block is smaller than or equalto 32×32, transform type candidate 0 is applied to a transform block of4×4 encoded in the intra mode, and transform type candidate 1 is appliedto a transform block which does not satisfy the above conditions. On theother hand, when the size of the coding block is larger than 32×32,transform type candidate 0 is applied to a block of 4×4 or 8×8 encodedin the intra mode, and transform type candidate 1 is applied to atransform block which does not satisfy the above conditions.

The transform type candidate may include a transform skip indicatingthat no transform is performed. Depending on whether a transform skip isallowed, at least one of types or the number of transform typecandidates included in the transform set may be set differently. As anexample, if the transform_skip_enabled_flag indicating whether or not toallow the transform skip in a picture is 1, a transform set whichfurther including the transform skip as the transform type candidate maybe used, as shown in Table 2. On the other hand, iftransform_skip_enabled_flag is 0, a transform set which does not includethe transform skip as the transform type candidate may be used, as shownin Table 1.

TABLE 2 Transform Transform Transform Transform set Index candidates 0candidates 1 candidates 2 0 DST-VII DCT-II Transform skip 1 DST-VIIDST-I Transform skip 2 DST-VII DCT-VIII Transform skip

Transform types of a horizontal transform and a vertical transform ofthe current block may be the same, or transform types of the horizontaltransform and the vertical transform may be different from each other.For example, a transform type candidate in the transform set may beapplied to both the horizontal transform and the vertical type, or adifferent transform type candidate may be applied to each of thehorizontal transform and the vertical type.

As another example, transform sets for the horizontal transform and thevertical transform of the current block may be the same, or transformsets of the horizontal transform and the vertical transform may bedifferent from each other. When different transform sets are used forthe horizontal transform and the vertical transform, a transform setindex for identifying the transform set for the horizontal transform anda transform set index for identifying the transform set for the verticaltransform may be individually signaled.

For example, a transform set corresponding to index 0 may be used forthe horizontal transform, and a transform set corresponding to index 1may be used for the vertical transform. If the current block is 4×4encoded with the intra prediction, the vertical transform and thehorizontal transform may use the transform type candidate 1 included ineach transform set. Accordingly, DST-II may be used for the horizontaltransform and DST-I may be used for the vertical transform.

It may be determined whether to use the same transform set for thehorizontal transform and the vertical transform depending on an intraprediction mode of the current block. For convenience of explanation,the transform set for the horizontal transform will be referred to as ahorizontal direction transform set, and the transform set for thevertical transform will be referred to as a vertical direction transformset.

For example, when the intra prediction mode of the current block issimilar to a horizontal direction or similar to a vertical direction,the horizontal transform and the vertical transform may use differenttransform sets. Here, the intra prediction mode similar to thehorizontal direction may include at least one of the vertical directionor intra prediction modes in which a difference in mode value from theintra prediction mode of the vertical direction is less than apredefined value. In addition, the intra-prediction mode similar to thevertical direction may include at least one of the horizontal directionor intra prediction modes in which the difference in mode value from theintra prediction mode of the horizontal direction is less than apredefined value. On the other hand, when the intra prediction mode ofthe current block is a non-directional mode or a directional mode whichdoes not satisfy the above condition, the vertical transform and thehorizontal transform may use the same transform set. Alternatively, itis also possible to use different transform sets for the verticaldirection and the horizontal transform of the current block when theintra prediction mode of the current block is the non-directional mode.

FIG. 9 is a diagram illustrating, for 33 intra prediction modes, whethera vertical transform and a horizontal transform use the same transformset. In the example shown in FIG. 9, it is depicted that the verticaland horizontal transforms use different transform sets when the intraprediction mode of the current block is included in a range of 7-13 or23-29. On the other hand, it is depicted that the same transform set isapplied to the vertical transform and the horizontal transform when theintra prediction mode of the current block is a directional mode notincluded in the above range.

If there exists a block having the same intra prediction mode as thecurrent block in a predetermined unit block, the transform set of thecurrent block may be set to be the same as the transform set of theblock having the same intra prediction mode as the current block. Here,the predetermined unit block may be a coding block, a coding tree block,or a block having a predetermined size.

For example, it will be assumed that an intra prediction modecorresponding to a first transform block in a scanning order in a codingblock has a vertical direction (for example, mode number 26), ahorizontal direction transform set of the block is index 2, and avertical direction transform set of the block is index 0. If there ismore transform block having an intra prediction mode of the verticaldirection in the coding block (i.e., a transform block corresponding toa prediction block having the intra prediction mode of the verticaldirection), a transform set index value is not signaled for the newlyscanned transform block. Instead, the transform set of the previouslyscanned transform block having the intra prediction mode of verticaldirection is applied as a transform set of the newly scanned transformblock. That is, a horizontal direction transform set of the newlyscanned transform block is determined as the index 2, and a verticaldirection transform set is determined as the index 0.

As another example, when there is a block having an intra predictionmode similar to the current block in a predetermined unit block, thetransform set of the current block may be set to be the same as thetransform set of the block having the intra prediction mode similar tothe current block. Here, the intra prediction mode similar to thecurrent block may include intra prediction modes within a predeterminedrange from a reference intra prediction mode. For example, when thereference intra prediction mode is a horizontal direction or a verticaldirection, the reference intra prediction mode and the intra predictionmodes within ±a from the intra prediction mode of the horizontaldirection or the vertical direction may be determined to be mutuallysimilar.

For example, it will be assumed that an intra prediction modecorresponding to a first transform block in a scanning order in a codingblock has a vertical direction (for example, mode number 26), ahorizontal direction transform set of the block is index 2, and avertical direction transform set of the block is index 0. When thereexists a transform block having an intra prediction mode similar to thevertical direction (e.g., mode number 27) in the coding block (i.e., atransform block corresponding to a prediction block having the verticalintra prediction mode), a transform set index value may not be signaledfor the newly scanned transform block. Instead, a transform set of thetransform block having the intra prediction mode that is similar to theintra prediction mode of the current block may be applied as thetransform set of the newly scanned transform block. That is, ahorizontal direction transform set of the newly scanned transform blockis determined as the index 2, and a vertical direction transform set maybe determined as the index 0.

At least one of a horizontal direction transform set or a verticaldirection transform set of the current block may be determined based onan intra prediction mode of the current block. For example, Table 3shows an example in which a fixed transform set index is assignedaccording to the intra prediction mode of the current block.

TABLE 3 Intra Mode 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 H 2 1 0 10 1 0 1 0 0 0 0 0 1 0 1 0 1 V 1 1 0 1 0 1 0 1 2 2 2 2 2 1 0 1 0 1 IntraMode 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 H 0 1 0 1 0 1 22 2 2 2 1 0 1 0 1 0 V 0 1 0 1 0 1 0 0 0 0 0 1 0 1 0 1 0

When the current block is encoded with the inter prediction, apredefined transform set may be used for the current block. For example,if the current block is encoded with the inter prediction, a transformset corresponding to index 0 may be used for the current block.

Alternatively, when the coding block is encoded with the interprediction, a transform set is selected for the coding block, andtransform blocks included in the coding block may use transform typecandidates included in the transform set of the coding block. At thistime, the transform type of each transform block may be determined by asize or a shape of the transform block, or information for identifyingthe transform type selected for each transform block may be signaledthrough the bitstream.

The determination of at least one of a plurality of transform typecandidate groups as the transform type of the current block may bedefined as AMT (Adaptive Multiple Transform). The adaptive multipletransform (AMT) may be applied to a coding block of a specific size or acoding block of a specific shape. At this time, information on the sizeor the shape of the coding block to which the adaptive multipletransform can be applied may be signaled through the bitstream. Here,the information on the size of the coding block may indicate at leastone of a maximum size or a minimum size. In addition, the informationmay be signaled through at least one of a block level, a slice header,or a sequence header.

Different transforms may be selectively used based on differentsize/shape in a unit of a slice or a block.

For example, in slice 0, DST may be used when the transform block isencoded in the intra prediction mode and a size of the transform blockis 4×4, and DCT may be used in other cases. In slice 1, DST may be usedwhen the transform block is encoded in the intra prediction mode and asize of the transform block is less than or equal to 8×8, and DCT may beused in other cases.

Different transforms may be selected based on at least one of an intraprediction mode and the number of samples in the transform block.Specifically, for example, when the transform block is encoded in theintra prediction mode and the number of samples in transform block is 16or less, the transform may be performed using DST, and DCT may be usedin other blocks.

Specifically, for example, when the transform block is encoded in theintra mode and the transform block is 2×8 or when the transform block isencoded in the intra mode and the transform block is 8×2, DST (DiscreteSine Transform) is used, and DCT-II (Discrete Cosine Transform) is usedin other blocks.

At this time, a syntax, different type transform block selectionindicator, indicating the number of samples in a block which is used asa reference for selecting different transforms may be signaled in aslice header or a picture parameter set.

Conditions for selecting transform type candidate 0 and conditions forselecting transform type candidate 1 may differ in a unit of a sequence,a slice, or a block. For example, in slice 0, the transform typecandidate 0 is selected only for a transform block of 4×4 encoded in theintra mode, while in the slice 0, the transform type 0 is selected for atransform block of 8×8 or smaller encoded in the intra mode.

Alternatively, the transform type may be adaptively selected based on atleast one of an intra prediction mode or the number of samples in ablock. At this time, the number of samples in the block used as areference for selecting the transform type may have a fixed value or maybe determined through information signaled through the bitstream. Theinformation may be signaled via a block level, a slice header, or apicture parameter set.

For example, DST may be applied only when the current block comprises 16or less samples and when the current block is encoded in the intra mode,and DCT may be applied in other cases. Specifically, DST may be appliedto a transform block of 4×4, 2×8 or 8×2 encoded in the intra prediction,and DCT may be applied to other blocks.

Although the above-described embodiments have been described on thebasis of a series of steps or flowcharts, they do not limit thetime-series order of the invention, and may be performed simultaneouslyor in different orders as necessary. Further, each of the components(for example, units, modules, etc.) constituting the block diagram inthe above-described embodiments may be implemented by a hardware deviceor software, and a plurality of components. Or a plurality of componentsmay be combined and implemented by a single hardware device or software.The above-described embodiments may be implemented in the form ofprogram instructions that may be executed through various computercomponents and recorded in a computer-readable recording medium. Thecomputer-readable recording medium may include one of or combination ofprogram commands, data files, data structures, and the like. Examples ofcomputer-readable media include magnetic media such as hard disks,floppy disks and magnetic tape, optical recording media such as CD-ROMsand DVDs, magneto-optical media such as floptical disks, media, andhardware devices specifically configured to store and execute programinstructions such as ROM, RAM, flash memory, and the like. The hardwaredevice may be configured to operate as one or more software modules forperforming the process according to the present invention, and viceversa.

INDUSTRIAL APPLICABILITY

The present invention may be applied to electronic devices which is ableto encode/decode a video.

The invention claimed is:
 1. A method for decoding a video, the methodcomprising: obtaining a residual coefficient of a current block; inversequantizing the residual coefficient; determining whether aninverse-transform is skipped for the current block; and obtaining aresidual sample of the current block by applying or skipping theinverse-transform for the current block, wherein when it is determinedthat the inverse-transform is not skipped for the current block,obtaining the residual sample comprises: determining a transform set forthe current block; determining a transform type of the current blockamong transform type candidates included in the transform set; andperforming the inverse-transform based on the determined transform type,and wherein skipping the inverse-transform for the current block is notallowed when the current block is partitioned into vertically orhorizontally.
 2. The method of claim 1, wherein the transform set of thecurrent block is selected among a plurality of transform set candidates,and wherein a type or a number of transform type candidates included inone of the transform set candidates is different from another of thetransform set candidates.
 3. The method of claim 1, wherein the inversetransform comprises a horizontal inverse transform and a verticalinverse transform, and wherein a transform set for the horizontaltransform and a transform set for the vertical transform are determinedindependently.
 4. The method of claim 3, wherein the transform set forthe horizontal inverse transform and the transform set for the verticalinverse transform are determined according to an intra prediction modeof the current block.
 5. The method of claim 1, wherein the transformtype of the current block is adaptively determined based on at least oneof a size, a shape or a number of samples of the current block.
 6. Amethod for encoding a video, the method comprising: obtaining a residualsample a current block; determining whether a transform is skipped forthe current block; obtaining a residual coefficient of the current blockby applying or skipping the transform for the current block; andquantizing the residual coefficient of the current block, wherein whenit is determined that the transform is not skipped for the currentblock, obtaining the residual coefficient comprises: determining atransform set for the current block; determining a transform type of thecurrent block among transform type candidates included in the transformset; and performing the transform based on the determined transformtype, and wherein skipping the transform for the current block is notallowed when the current block is partitioned into vertically orhorizontally.
 7. The method of claim 6, wherein the transform set of thecurrent block is selected among a plurality of transform set candidates,and wherein a type or a number of transform type candidates included inone of the transform set candidates is different from another of thetransform set candidate.
 8. The method of claim 6, wherein the transformcomprises a horizontal transform and a vertical transform, and wherein atransform set for the horizontal transform and a transform set for thevertical transform are determined independently.
 9. The method of claim8, wherein the transform set for the horizontal transform and thetransform set for the vertical transform are determined according to anintra prediction mode of the current block.
 10. The method of claim 6,wherein the transform type of the current block is adaptively determinedbased on at least one of a size, a shape or a number of samples of thecurrent block.
 11. An apparatus for decoding a video, the apparatuscomprising: an entropy decoding unit to decode a residual coefficient ofa current block; an inverse quantization unit to inverse quantize theresidual coefficient; and an inverse transform unit to determine whetheran inverse-transform is skipped for the current block, and to obtain aresidual sample of the current block by applying or skipping theinverse-transform for the current block, wherein when it is determinedthat the inverse-transform is not skipped for the current block, theinverse transform unit is further configured to: determine a transformset for the current block, determine a transform type of the currentblock among transform type candidates included in the transform set, andperforming the inverse transform based on the determined transform type,and wherein skipping the inverse-transform for the current block is notallowed when the current block is partitioned into vertically orhorizontally.